The invention is based on a molded piston, in particular an injection-molded piston of fiber-reinforced plastic.
One such piston is known from German Patent DE 42 33 896 C2. The known piston is received slidably in a receiving bore of a housing and embodied as a cup-shaped body with a piston bottom and a sleevelike piston wall protruding away from it. Between the face of the piston bottom pointing away from the piston wall and the bottom of the receiving bore, a pressure chamber is formed, which when pressure is exerted on it increases in size, counter to the force of a piston spring that is prestressed counter to the piston.
The known piston is produced by injection molding in an injection-molding tool, which is likewise cup-shaped and on its bottom has an injection-molding channel for pressure-injection of a thermoplastic plastic into a chamber between the inner wall of the tool and a cylindrical core; this interstice, filled with injection-molding material under pressure, is shaped in accordance with the geometry of the piston. In the region of the so-called injection point of the molded piston, that is, opposite the outlet opening of the injection-molding channel, an injection cone forms on the face of the piston bottom on which pressure is exerted in operation, and this injection cone is troublesome in the sense that once the piston has been inserted into the receiving bore of the housing, the injection cone protrudes into the pressure chamber and must therefore be removed, for instance by cutting-type machining, which entails a certain amount of production effort and expense.
From U.S. Pat. No. 5,031,511, a piston of this generic type is known, having a peg formed centrally onto its bottom; the peg protrudes from the face that points away from the pressure-loadable face. A pivotable thrust piece for pressing a brake lining against a brake disk of a disk brake is received on the peg. The bottom of the piston is provided, toward the peg, with a rounded recess of low depth, which extends radially spaced apart from the peg and creates a free space into which the thrust piece can move upon a pivoting motion. U.S. Pat. No. 5,031,511 discloses nothing about the location of the injection point of the piston.
In International Patent Disclosure WO-A 97/18403, FIG. 7, a molded piston is disclosed that has an axially deep encompassing recess, which originates on the face end of the piston that points away from the pressureloadable face. The recess defines a peg, which like the recess and the face end of the piston is enveloped by a sheet-metal part that is indented in the injection molding of the piston. Consequently, an injection point can be located only in the region of the pressure-loadable face or of the piston jacket.
Accordingly, in the invention, the injection point is shifted to the other side, pointing away from the pressure-loadable face on the piston bottom, because in that location, the injection cone is not troublesome. Since from pressure exerted on the pressure chamber the piston bottom is a plate substantially stressed by bending, in operation on the one hand a region subject to compressive stresses develops in a known way, extending from the pressure-loadable face of the piston bottom as far as a neutral fiber in the interior of the piston bottom; on the other, a region subject to tensile stresses develops, which extends from the neutral fiber up to the face of the piston bottom that points away from the pressure-loadable face. If the injection point were not shifted to the face of the piston bottom pointing away from the pressure-loadable face, without any other provisions being made, then the injection point would be located at a point where the tensile stresses are greatest. Since the injection point of an injection-molded part is known to be shot through with bubbles and porosities, however, the strength properties there are critical. Another factor is that in injection-molding materials, as a rule the maximum tensile strength is less than the maximum pressure strength, so that under the high pressures that such a piston can be exposed to in operation, cracks and breaks can occur on the piston bottom.
As a result of at least one recess on its face pointing away from the pressure-loadable face of the piston bottom, the piston bottom has a lesser wall thickness in the region of the injection point than in the regions adjoining it. Since location of the neutral fiber of the entire piston bottom is determined essentially by the radially outer regions, adjoining the middle region of the injection point, the wall region of the piston bottom located on the side of the tensile stress is removed entirely or in part by means of the at least one recess in the region of the injection point. Accordingly, as a result of the recess in the region of the injection point that is critical in terms of strength, the wall region subjected to tensile is reduced in size, so that the remainder is free of tensile stress or predominantly pressure relieved, which has a favorable effect on the strength properties.
In order to avoid the tensile stresses in the piston bottom in the vicinity of the injection point, which are unfavorable in terms of strength, it is proposed in accordance with an especially preferred embodiment that the at least one recess is embodied as deep enough that the piston bottom face pointing away from the pressure-loadable face, extends from the direction of the pressure-loadable face, in the region of the injection point, at most as far as a plane that includes the neutral fiber of the regions of the piston bottom that adjoin the region of the injection point. If there is coplanarity between this plane that includes the neutral fiber and the face of the piston bottom pointing away from the pressure-loadable face, then the tensile stresses in this face are equal to zero, while in the case of relatively deep recesses, this face is subjected solely to pressure.
In the preferred embodiment, the at least one recess is embodied annularly and surrounds a truncated cone or feeder pointing away from the piston bottom and coaxial with the piston axis, on the free end of which truncated cone or feeder the injection point is located. Because of the centrally-disposed truncated cone, the strength-critical porosities are located in a substantially stress-free region. Moreover, this creates an additional volume, which can absorb any microstructural flaws, caused by erroneous injection-molding parameters, in a virtually stress-free region. Alternatively, the at least one recess can be a fully cylindrical recess disposed coaxially with the piston axis.